He said that in the past a single, wired access point was registered to a single person and could only be used by one person at any given time.

Now wireless routers stuff up the logic that an IP address could be a person. Since 61 percent of US homes now have wireless access it is possible that a single IP address usually supports multiple computer devices which could be operated by different individuals.

Neighbours or passersby could access the internet using the IP address assigned to a particular subscriber and download a plaintiff’s film.

It sounds obvious but so far it has avoided being presented as a serious argument in any court case.

But the ruling will mean that Big Content will have to come up with a lot more proof of who the pirate is, rather than an IP address.